This is what prompted my post.What I tried and seems to work is an old surge /spike supressor plug from the late 50's 60's plugged into the unused outlet.
Since this helps to tame some extraneous noise,and is so simple,I wouldn't doubt that the lamp would also do something good, I'll have to give it a go.
Perhaps some of the more technically oriented folks could give a reason why plugging some sort of apparatus other than audio gear into an unused portion of a receptacle, seems to make an audible difference.
I have heard of some folks hard wiring a cap or resistor across their receptacles, maybe they could illuminate us.
Maybe all dual receptacles need some kind of load on each receptacle to soak up some noise on the line or at least balance the noise between the two receptacles.
Just speculating, but leaving one receptacle open doesn't sound as good,until I tried the surge plug I never would have thought about any of this even mattering.